1. Field of the Invention
The present invention relates to a rotary clipper for clipping a traveling sheet of thin material into sections and more particularly to a drive for a rotary clipper especially suited for close-interval clipping of wood veneer.
2. Description of the Prior Art
As a sheet of veneer is peeled from a log chucked in a veneer lathe, the traveling sheet is clipped at intervals by a veneer clipper into sections of random or predetermined lengths for convenience in further handling as the sheet proceeds toward the veneer dryer. It is also common practice to remove defects from the traveling sheet by having the clipper clip it twice in rapid succession, once on either side of the defect, to remove a narrow strip containing the defect from the sheet. This is done automatically using a scanning device upstream of the clipper to detect the position and size of a break in the traveling sheet and to transmit corresponding signals to the clipper to trigger its operation twice in succession at the appropriate time and interval to remove the defective strip from the sheet.
In the interest of minimizing material waste, it is desirable in removing defects to make the clipping interval as close as possible so that only a narrow strip just wide enough to contain the defect is removed. However, to remove a narrow strip from the traveling sheet requires either extremely accurate, high-speed, close-interval clipping or slow-speed sheet travel, or a compromise between the two. Existing clippers are not capable of close-interval clipping at high sheet speeds. As a result, existing veneer clippers are a bottleneck in the veneer manufacturing process.
A problem in clipping veneer that limits clipping and sheet speeds is the tendency of veneer to curl at its clipped ends, particularly veneer peeled from near the core of a log. The curled end edges tend to hang up on the clipper blade, causing bunching, jamming and breakage of the fragile veneer at the blade. When this occurs, the production line must be shut down and the jam and broken material cleared, resulting in a loss of valuable production time and material.
Veneer clippers in common use are of the reciprocative guillotine type in which a single blade is driven vertically downwardly through a traveling sheet and then retracted to clip the sheet into sections. Since the reciprocatory movement of the guillotine blade in this type of clipper is inherently slow and is not in the direction of travel of the sheet, the tendency of the sheet to bunch, jam and break at the blade is particularly great, requiring slow sheet travels speeds not exceeding about 180 feet per minute. Even at such slow speeds, clippers of this type cannot clip out a defective strip less than about 31/2 inches wide.
A rotary-type veneer clipper has been proposed, as shown in U.S. Pat. No. 3,808,925, issued May 7, 1974. However, no rotary clippers are known to be in commercial use. The proposed such clipper is characterized by a single thin blade sharpened at both opposite edges to provide two 180.degree. spaced-apart cutting edges, one of which rotates into cutting cooperation with a roller anvil to clip a sheet of veneer moving across the anvil upon each 180.degree. rotation of the blade about its central axis. Since the blade cutting edge moves as it clips in the same direction as the sheet, the problems of bunching, jamming and sheet breakage should be reduced.
However, a rotary clipper has another problem which arises from the inertia of rotary knife movement and the necessity of stopping knife rotation after each clipping stroke to await the next clip signal. To meet this inertia problem, the two-edge blade of the aforementioned proposed rotary clipper is made of very thin lightweight material which requires use of a longitudinally pretensioned blade and a bracing roll which is engaged by one of the two cutting edges to prevent blade deflection as the other cutting edge contacts the roller anvil to make the cut. Thus in such a rotary clipper, the spacing between the anvil roll, blade and backup roll becomes critical. In practice it is suspected that precise adjustments would have to be provided between these three elements for effective operation.
Previously proposed rotary clippers are also limited in their attainable clipping speeds by the nature of their driving mechanisms. The clipper drive is coupled to the drives for the sheet conveyor and the anvil and bracing rolls so that clipper speed is synchronized with, and limited by, sheet and roll speeds.
Previously proposed rotary clippers as described also propose to use clutch and brake means to start and stop knife movement. This can lead to inaccuracy in indexing of the clipper blades for making the next cut after each cutting stroke. Inaccurate indexing in turn leads to inaccurate timing and thus inaccurate clipping, the effects of which are amplified at high sheet speeds.
Although not a veneer clipper, a device known as a rotary sheet breaker has been used to break a traveling ribbon of veneer into sheets of convenient length for further processing as it is peeled from the log. Such a breaker is characterized by a single shaft-mounted breaker knife which when activated by a manual control rotates through 360.degree. into cutting cooperation with a roller anvil to break the sheet and return to its starting position. A drive mechanism including a clutch and brake is used to start and stop rotation of the breaker knife. Because of its 360.degree. cutting cycle and the inherent limitations in speed and accuracy of the clutch and brake drive, such a sheet breaker is unsuitable for use as a veneer clipper.
Other rotary shearing or clipping devices have been suggested but they are not suitable for veneer clipping. For example, U.S. Pat. No. 3,111,875 discloses a rotary cutting device for cutting fibers which rotates continuously during operation and is therefore unsuitable for use as a veneer clipper.
The apparatus of U.S. Pat. No. 3,677,120 is representative of fluid cylinder-driven rotary shears using an interacting pair of rotary knives and developing high cutting forces for cutting slowly moving thick steel sheet material. The drive cylinder is capable of operating the shear only during its extension stroke. Such rotary shears, because of their slow operating speeds, slow sheet speeds and dual knife construction, are unsuited for use as veneer clippers.
From the foregoing it will be apparent that there is a need for a high-speed veneer clipper capable of accurate, close-interval clipping of defects from sheets of veneer traveling at high speed while minimizing the aforementioned problems of bunching, jamming and sheet breakage common with existing veneer clippers.